This invention is generally related to metal oxide semiconductor (MOS) digital drivers, and more particularly to circuitry that protects the driver against overshoot/undershoot voltages at the driver output node when the driver is not transmitting.
A high voltage driver can translate a digital signal from a given voltage range to a greater voltage range. An electronic device, such as a modern processor built in a MOS integrated circuit die, may use tens or even hundreds of MOS drivers, one for each of its signals, to communicate with another device or bus. High density devices such as advanced processors use a relatively low supply voltage which permits greater on-chip computing performance and lower power dissipation, both very desirable results. However, higher voltages are often needed by other types of devices and for reliable bus signaling. The high voltage driver provides the voltage translation to interface between low and high supply voltage devices.
Typically, a device uses drivers that are built on its own die to transmit and receive the high voltage signals from the second device. Certain MOS field effect transistors (MOSFETs) at the output stage of the driver are capable of operating reliably at a higher supply voltage than the rest of the die so that they may generate and withstand the higher voltages associated with the second device. In addition, these transistors also withstand voltage overshoots and undershoots (beyond the normal supply range) that appear at the output node of the driver in receive mode, when the driver is not transmitting.
In one conventional technique, the exposed driver transistors are specially designed and built to be different than the other xe2x80x9cstandardxe2x80x9d transistors in the die, so that they may withstand the higher voltages needed to communicate with the second device. For instance, a special driver MOSFET can be designed to have a thicker gate oxide than its standard siblings on the same die. Such a solution, however, significantly increases the manufacturing cost of the die because some fabrication steps need to be modified and/or added to build the special transistors.
Another limited solution is to use standard transistors for the drivers, and add a pair of diodes to limit the output node voltage of each driver. In the presence of an undershoot, for instance, the node voltage is clamped to no lower than one diode drop (approximately 0.7 Volts) below the lower supply node voltage (e.g. ground). However, a 0.7 Volt diode drop may stress or damage the relatively thin gate-oxide of standard MOS transistors that are typically built using advanced fabrication processes for operation at very low supply voltages. In these fabrication processes, the gate oxide of a standard MOSFET may tolerate a small voltage, e.g. for a supply of 1.8 Volt, the maximum voltage across the gate oxide may be no more than 0.27 Volts greater than the 1.8 Volt supply. Thus, diodes by themselves are not suitable to protect high voltage drivers built with standard transistors in such processes.
Yet another limited solution uses a clamp consisting of a pair of MOSFETs biased in their sub-threshold regions of operation, rather than the diodes. Although the sub-threshold biased MOSFETs can provide enough protection only when their widths are large enough to prevent voltage build up at their drain causing electrical-over-stress, a large device biased at sub-threshold will introduce a significant amount of leakage current which may not be desirable in many applications. In addition, their sophisticated bias circuits may also consume a relatively large amount of on-chip area.